(1) Field of the Invention
The present invention relates to processes used to create semiconductor devices, and more specifically to a process used to self-align a source region, to a polysilicon gate structure, followed by the creation of a field oxide, (FOX), region.
(2) Description of Prior Art
One method for fabricating flash EPROM, (electrical programmable read only memory), devices, entails the use of a common source region. Prior art, such as Tang, et al, in U.S. Pat. No. 5,120,671, describes a fabrication sequence for a EPROM type device, using a common source region. In that invention the common source region is formed after the creation of both a polysilicon gate structure, and isolating field oxide, (FOX), islands. The process sequence used in this prior art can result in an unwanted corner rounding sequence of the FOX structures, resulting in less available area remaining for the subsequent source region. Thus designs are used, allowing additional area for the source region, however adversely influencing the trend to improved device density. The process sequence used in the cited prior art can also result in semiconductor substrate undercutting, during the removal of unwanted FOX regions, in regions to be used for the common source region. The unwanted substrate undercut can result in low breakdown voltages.
This invention will describe a process sequence in which a self-aligned source region, for a subsequent semiconductor memory device, such as an EPROM type device, is formed after creation of a stacked gate structure, but before formation of the isolating FOX regions. This sequence places FOX regions only in desired locations, eliminating the need for removal of unwanted FOX regions, thus eliminating the possible damage created in the semiconductor substrate, during the FOX removal procedure, encountered with prior art. In addition the creation of the FOX region, post stacked gate formation, results in birds beak formation, under the stacked gate structures, decreasing a capacitance component, which enables an increase in the coupling ratio, of the EPROM type device, to be realized.